Such devices for feeding a fluid to rotating machine part, also called "rotary lead-throughs", are known in the most diverse types and for the most diverse purposes (see Catalogue 825D "DEUBLIN Rotating UNIONS" issued by Deublin GmbH, D-6238 Hofheim-Wallau, pages 27 and 28). Such devices are employed, for example, in conjunction with machine tool spindles, the outer end of the hollow shaft being fixedly connected to the machine tool spindle or a pull rod, arranged centrally therein, of a clamping facility installed in the machine tool spindle. In this case, the device serves to feed a cooling lubricant to the tool. As long as the feed of the said fluid takes place during the rotation of the hollow shaft connected to the machine tool spindle or the clamping facility, no problems occur at the sliding ring seal, since this is lubricated by the fluid and the frictional heat produced is removed. If, however, dry machining of the workpiece without cooling lubricant is to take place, then the conventional rotary lead-throughs are not suitable for this purpose. In fact, if the cooling lubricant is absent at the sliding ring seal, then this seal is rapidly destroyed, especially at higher spindle speeds, because of lack of lubrication, but in particular because of lack of cooling. Special rotary lead-throughs have therefore been developed in which, even in the absence of feedthrough of a fluid, the cooling and lubrication of the sliding ring seal is ensured.
In a known rotary lead-through for this purpose (DE 35 42 014 C1), in the case of dry machining (dry operation), i.e. when no fluid is conveyed through the rotary lead-through, one sliding ring of the sliding ring seal can be lifted away from the other sliding ring. In the known rotary lead-through, this takes place owing to the fact that one sliding ring is arranged on a hollow plunger which is axially slidable in the housing. The hollow plunger is acted on by a spring which loads the hollow plunger in the direction away from the other sliding ring. Moreover, a stop valve spring-loaded in the direction opposite to the direction of flow of the fluid is arranged in the hollow plunger. With the stop valve closed, the plunger is first displaced by the pressure of the fluid in opposition to the force of the plunger spring and the sealing surfaces of the two sliding rings are thereby brought into contact. Only then does the stop valve open and release the flow. If, however, in dry operation, the sealing surfaces are lifted away one from the other, particles of dirt contained in the fluid, in particular the cooling lubricant, which is used again and again in the circuit, may get between the sealing surfaces. In sliding ring seals, the sealing surfaces show great flatness and are very hard. If, with the sliding ring lifted away, a small foreign body gets between the sealing surfaces and the sliding rings are thereafter pressed against one another again, this foreign body leads to rapid destruction of the sliding ring seal. In this way, with slightly contaminated fluid, the life of the sliding rings is very much shortened. Moreover, even in dry operation, in which no cooling lubricant at all has to be conducted through the lead-through, the hollow shaft rotates together with the machine tool spindle. In particular at high speeds of rotation of the spindle, this leads to unnecessary wear of the ball bearings by means of which the hollow shaft is mounted in the housing. Moreover, noise and vibration may emanate from the rotary lead-through.
In another kind of rotary lead-throughs, the problem of lubrication and cooling of the sliding rings during dry operation is eliminated in that an auxiliary coolant and lubricant is fed to the sliding rings from outside. This, however, requires a not inconsiderable design expenditure, because the auxiliary coolant and lubricant must be fed to and removed from the region of the sliding rings continuously, so that the frictional heat generated at the sliding rings is continuously eliminated. In dry operation, needless wear of the ball bearings and the sliding rings and needless loss of efficiency occur. Moreover, the above-mentioned vibration and the nuisance caused by noise are also noticeable.
Therefore, the problem underlying the invention is to provide a device of the kind mentioned at the beginning for feeding a fluid to a rotating machine part, in particular a machine tool spindle, in which in the absence of feedthrough of a fluid (dry operation) the sliding rings do not have to be lifted one away from the other and nevertheless wear thereof, bearing wear and vibration are avoided, and which does not require any separate cooling arrangement.
According to a first solution proposed by the invention, this is achieved in that a coupling tube coaxial with the hollow shaft and connected to it to be fixed against rotation is arranged to be axially slidable in the outer end thereof and the coupling tube is designed as a hollow plunger slidable in the outer end of the hollow shaft, is movable under the pressure of the fluid towards the rotating machine part in opposition to the force of a return spring and can be engaged with this machine part with a sealing arrangement interposed, in such manner that, in operation with fluid, the coupling tube can be engaged with the machine part by axial movement towards it and, in operation without fluid, can be disengaged from the machine part by axial movement away from it.
According to the invention, a second solution of the above-mentioned problem consists in that a coupling tube coaxial with the hollow shaft and connected fixedly to it is provided at the outer end thereof, the housing is movable axially with respect to the rotating machine part under the action of a servomotor and the coupling tube can be engaged with the rotating machine part with a sealing arrangement interposed, in such manner that, in operation with fluid, the coupling tube can be engaged with the machine part by axial movement of the housing towards the latter and, in operation without fluid, can be disengaged from the machine part by axial movement of the housing away from it.
Thus, in each case, the invention starts from the idea of disengaging the entire rotary lead-through from the rotating machine part in dry operation, i.e. when no fluid is to be fed to the rotating machine part. In the first proposed solution, the disengagement takes place owing to the fact that when the feed of the fluid is interrupted, the coupling tube telescopically slidable in the outer end of the hollow shaft is moved away from the rotating machine part by the force of the return spring. Engagement takes place owing to the fact that the coupling tube is moved towards the rotating machine part under the pressure of the fluid in opposition to the force of the return spring. In the second proposed solution, engagement and disengagement take place by axial movement of the entire housing containing the rotary lead-through by means of a servomotor. In both cases, when no fluid is to be fed to the rotating machine part, the entire rotary lead-through is disengaged from this part, which has several advantages. While, in dry machining, the machine tool spindle rotates, the disengaged hollow shaft is at rest. In consequence, neither wear of the sliding rings nor bearing wear can occur. Vibration is also avoided. An auxiliary cooling facility for cooling the sliding ring seals can be dispensed with. Lifting away of sliding rings in dry operation is likewise unnecessary and the disadvantages involved thereby are also avoided. All in all, the rotary lead-through has a long life, because it is completely out of action in dry operation.
A device for feeding a fluid which is under pressure to a rotating machine part is also known (FR 1 505 040) in which a hollow shaft is mounted rotatably and to be axially slidable in opposition to the force of a spring in a stationary housing. A part of the hollow shaft is in the form of a plunger. The sealing action between the rotating plunger and the stationary housing takes place through the medium of cylindrical sealing surfaces between the plunger and a cylindrical bore provided for the purpose in the housing. The clearance between the plunger and the cylindrical bore should be as small as possible in order to achieve a seal between the two parts. This requires a very high accuracy of manufacture. Even with slight wear, the sealing action diminishes considerably, since the plunger surfaces are not pressed resiliently against the cylindrical bore. Because of the considerable wear to be expected, this sealing method is also only usable for low speeds of rotation. In this known device, the axial movement of the hollow shaft is effected in that a valve arrangement with a flow bore which is eccentric with respect to the axis of rotation is provided in the plunger. The surface surrounding the valve seat is of conical form, so that with a perpendicularly extending axis of rotation, with the hollow shaft stationary, a valve ball rolls at any given time to the lowest point of the conical surface and closes the flow bore provided thereat. If a fluid under pressure is fed to the plunger, it and, consequently, the entire hollow shaft is shifted axially in opposition to the force of the spring. As a result of this axial movement, the outer end of the hollow shaft is engaged with a rotating machine part. The hollow shaft is thereby set in rotation and the eccentrically acting valve ball is acted on by centrifugal forces which move the valve ball radially outwards and lift it away from the valve seat. As a result of this, the flow is then opened up. In consequence of the described valve arrangement, the conveying through of fluid is possible only in the case of rotation. This publication has not anticipated the present invention, because if the cylindrical sealing surfaces were to be replaced by a sliding ring seal with axial sliding surfaces, these would lift away one from the other on axial movement of the hollow shaft and the same disadvantages would arise as are also present in the rotary lead-through according to the above-mentioned DE 35 42 014 C1.
In another known rotary lead-through (GB 1 365 640), the rotating machine part forms a unit with the hollow shaft. A transfer tube is provided in the hollow shaft, the tube being pressed against the rotating sliding ring by a spring and thrusting the ring against the stationary sliding ring. Owing to this, axial pressing of the sliding surfaces into contact takes place. The transfer tube is pressed constantly against the rotating sliding ring and serves to carry through a first fluid, which passes through central bores in the two sliding rings. Parallel to the central bores, the sliding rings have several flow bores arranged on a common circle, through which a second fluid can flow. This second fluid then flows further through an interspace formed between the transfer tube and the interior of the hollow shaft. Since the transfer tube serves both to carry through a fluid and for axial pressing of the sliding ring surfaces into contact, it is constantly pressed against the stationary sliding ring by the pressure spring. It cannot be disengaged from the body rotating with it or from the rotating sliding ring, except by dismantling of the entire rotary lead-through. In order to avoid unnecessary wear, it is proposed in this publication to remove the connecting pieces which serve to feed the fluids in a normally stationary housing part from this stationary housing part, so that this stationary part can join in the rotation. The removal of the connecting pieces, however, is a time-consuming and complicated matter and the re-attachment of the connecting pieces likewise requires a rather great expenditure of time. Basically, it is true, the problem underlying the present invention is solved with this known rotary lead-through, but in a different and complicated manner. Since the transfer tube is not axially movable and also cannot be disengaged from any adjacent parts during the operation of the rotary lead-through and also when the same is at a standstill, this publication also could not anticipate the present invention for want of a prototype.